Question

Why 12 molecules of water is used in the equation of photosynthesis

Answer 1

12 molecules are not shown as the substrate. 6 molecules are.The final product of photosynthesis is a carbohydrate - CH(2)O, which is generally accepted as being glucose - C(6)H(12)O(6). Along with glucose, oxygen is also released as a by-product during the light dependent reactions.The light dependent reactions involve photolysis of water and the subsequent transfer of electrons, that catalyses the formation of ATP and NADPH.H(2)O + photons →2 H(+) + O(-) + 2e(-)The electrons are transfered to NADP(+) and thus are removed from the equation. Also, 3 ATP molecules are formed. Thus, the net reaction of the light dependent reaction is,H(2)O + 2 NADP(+) + 3ADP + 3P(i) + photons →2 H(+) + O(-) + 2 NADPH + 3 ATPThe light independent reaction consist of the Calvin cycle, followed by a pathway that converts G3P go a hexose sugar. The net equation of the Calvin cycle is,3 CO(2) + 6 NADPH + 9 ATP → G3P + 6 NADP(+) + 9 ADP + 8 P(i)Subsequently, glucose can be formed by combining 2 G3P molecules. To produce 2 G3P molecules, two cycles are required. So the net reaction for the light independent reactions is,6 CO(2) + 12 NADPH + 18 ATP → C(6)H(12)O(6) + 12 NADP(+) + 18 ADP + 16 P(i)By combining and balancing the two equations, we get,6 CO(2) + 6 H(2)O → C(6)H(12)O(6) + 6 O(2)NADP/NADPH and ADP/ATP cancel out